The Atlantic Niño weakens under global warming

The Tropical Atlantic Ocean is locked in between the Brazilian coastline on one side, and the west-African coastline in the east. As the ocean is a huge player in the climate systems, changes in the ocean affects the local weather – it also changes the marine ecosystems and affects people living of the fish resources within the oceans.

As in the large Pacific Ocean, the South Atlantic Ocean also have El Niño events occurring on interannual time scales.

In a new study published in Nature Climate Change, researchers investigate how the Atlantic Niño will be affected under global warming.

The Atlantic Niño effects:

• In the Gulf of Guinea: Warm sea-surface temperature anomalies related to Atlantic Niño cause increase of rainfall over Gulf of Guinea and decrease over the Sahel region. Opposite effect with Atlantic La Niña when the anomalies are negative.
• Fisheries, especially for the West African coastline: When there is weaker interaction between the deep layers and upper layers of the ocean, the upwelling of nutrients is expected to be less efficient, so the amount of available nutrients that are vital for marine ecosystems are expected to decrease.
• Rainfall in Brazil: Rainfall in Brazil is also influenced by the Atlantic Nino, but in a more complicated way as other phenomena also contribute to rainfall in this region.

Reduced variability of surface ocean temperatures

By investigating a large ensemble of climate models, and accounting for model errors, the researchers found a reduction in variability of surface ocean temperatures of as much as 24-48 per cent by the end of the century, under the highest emission scenarios.

According to lead author Lander Crespo, this study provides well-grounded indications of how the Atlantic Niño will change.

“We were positively surprised to be able to find robust changes in the future tropical Atlantic variability, since it is well known that climate models don’t do very well in this region. Therefore, no one has attempted to study climate changes in the area before," he says. "The projected weakening of the Atlantic Niño is large enough that we can expect large changes in the oceanic and atmospheric circulations with consequences for local fisheries as well as changes in the drought and rainfall patterns. Now we need to investigate how these changes will affect the climate system and, ultimately, the consequences for society.”

Crespo is a researcher at the University of Bergen and the Bjerknes Centre for Climate Research.

Weakened coupling between the deeper ocean layers and the surface

What happens, is that in a warmer future climate, the eastern equatorial Atlantic upper ocean warms faster than the deeper ocean. Therefore, the thermocline becomes deeper. The thermocline marks a boundary between temperatures above and under 20°C- 23°C.

With a deeper thermocline, the ocean surface become less sensitive to anomalies in the deeper ocean. These anomalies include upwelling of nutrients, a phenomenon important for the productivity in the marine ecosystems.

A deeper thermocline means that the ocean surface is farther away and less influenced by the cold water below. In other words, the surface waters are less sensitive to changes in the deeper layers. This effect is what the researchers call a weakening of the thermocline feedback.

“We propose the thermocline feedback weakening as the leading mechanism for the future changes in Atlantic variability,” Crespo says.

This effect is remarkably different to the driving mechanisms in the equatorial Pacific, the study shows.

“In the Pacific the changes in variability seem to be controlled by changes in the SST gradient between the east and the west part of the basin triggered by changes in the equatorial winds. While our mechanism is robust and presents a quite good model agreement, this doesn’t seem to be the case in the Pacific where the uncertainties are larger, and the models don’t agree that well; some models predict a warmer east Pacific while others predict colder anomalies," Crespo says.

Less variability makes predictions more difficult

A conclusion by the researchers in the study, is that less variability in the ocean, makes it harder to predict changes.

“When we say there are less variations, in the study, we talk about statistics over fifty years. Even if the overall variability is decreasing, we can still see strong El Niño events in the future with large impacts for people living along the West African coast,” Crespo says.

Reference:

Crespo et al. Weakening of the Atlantic Niño variability under global warming, Nature Climate Change, vol. 12, 2022. DOI: 10.1038/s41558-022-01453-y

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